Android - Where to get old LineageOS ROMs?

To answer the question (getting old builds) directly - there are people who foresaw the need and stashed some of the builds elsewhere: 1, 2.

As for the reason to it, it's right in their FAQ. The main reason would be to save bandwidth/storage, since the project is a non-profit.

You can try XDA-Developers to download Lineage OS for your device. But the ROMs posted are not official versions and might not always work as smoothly as the official. The version posted on the link is Lineage OS 16.